1. Field of the Invention
The present invention relates to a driving circuit for a flat display apparatus and a driving method using the same. More particularly, the present invention relates to a display apparatus which receives a digital video signal to produce a display image with gray scales in accordance with the received digital video signals.
2. Description of the Related Art
FIG. 1 shows a data driver in a conventional driving circuit for driving a display apparatus which displays multiple gray scale images in accordance with digital video signals. For simplicity of explanation, it is herein assumed that the digital video data consists of two bits (D.sub.0, D.sub.1). This data driver supplies driving voltages to N pixels (where N is a positive integer) on a scanning line which has been selected by means of a scanning signal.
FIG. 2 shows a circuit which is a part of the data driver of FIG. 1. This circuit, which is denoted by the reference numeral 20, supplies a driving voltage through a data line to the n-th pixel (where n is an integer of 1 to N) of the above-mentioned N pixels provided along the single scanning line. The circuit 20 includes sampling flip-flops 21 each for receiving one bit of the digital video data (D.sub.0, D.sub.1), holding flip-flops 22, a decoder 23, and four analog switches 24 to 27. To the analog switches 24 to 27, signal voltages V.sub.0 to V.sub.3 are respectively supplied from four different voltage sources. As the sampling flip-flops 21, D flip-flops or various other flip-flops can be used.
Hereinafter, the operation of the circuit 20 of FIG. 2 is described below. At a rising edge of a sampling pulse T.sub.smpn corresponding to the n-th pixel, the sampling flip-flops 21 get digital video data (D.sub.0, D.sub.1) and hold the digital video data therein. When such video data sampling for the 1st to Nth pixels on a signal scanning line is completed (i.e., sampling corresponding to one horizontal period is completed), an output pulse OE is applied to the holding flip-flops 22. Upon receiving the output pulse OE, the holding flip-flops 22 get the digital video data (D.sub.0, D.sub.1) from the sampling flip-flops 21, and transfer the digital video data to the decoder 23. The decoder 23 decodes each bit of the digital video data (D.sub.0, D.sub.1), and turns on one of the analog switches 24 to 27 in accordance with the respective values of the thus decoded bits. As a result, one of the signal voltages V.sub.0 to V.sub.3 from the four different voltage sources, which corresponds to the thus turned-on analog switch 24, 25, 26, or 27, is output from the circuit 20.
A conventional data driver such as described above requires 2.sup.n different voltage sources (where n is the number of bits constituting the digital video data). Accordingly, the number of required voltage sources doubles when the digital video data is enlarged by one bit. For example, in the case where the digital video data consists of 4 bits for displaying 16 gray scale images, the number of required voltage sources is: 2.sup.4 =16. Similarly, in the case where the digital video data consists of 5 bits for displaying 32 gray scale images, the number of required voltage sources is: 2.sup.5 =32. In the case of 6-bit digital video data for displaying 64 gray scale images, the number of required voltage sources is: 2.sup.6 =64.
Such voltage sources are connected through the analog switches of the data driver to a display panel, e.g., a liquid crystal panel, which provides a heavy load on the voltage sources. Thus, each voltage source is required to have a sufficient performance to drive such a heavy load. The increase in the number of high-performance voltage sources is a significant factor which causes the cost of the entire driving circuit to be high. Furthermore, since high-performance voltage sources cannot readily be placed within the LSI circuit of the driving circuit, the voltage sources must be located outside the LSI circuit. This means that signal voltages for driving the liquid crystal panel must be supplied from voltage sources external to the LSI circuit. As a result, with an increase in the number of voltage sources, the number of input terminals of the LSI circuit must be increased accordingly. It is extremely difficult to produce an LSI circuit having such a large number of input terminals. Even if it is possible to make such an LSI circuit, implementing or manufacturing problems arise in the mass production thereof; it is practically impossible to mass-produce such LSI circuits.
To solve the above-described problem, a driving method and a driving circuit using this method is disclosed in Japanese Laid-Open Patent Publication No. 6-27900. In the driving method and driving circuit, external voltage sources for supplying gray-scale reference voltages are used to generate a plurality of interpolated voltages, so that multiple gray scales can be obtained using both the gray-scale reference voltages and the interpolated voltages. This makes it possible to obtain multiple gray scales the number of which is larger than that of the voltage sources. Several types of data drivers using the driving method have been put into practical use.
FIG. 3 shows a circuit 30 which is a part of the data driver in the proposed driving circuit. According to the circuit 30, four interpolated voltages (V.sub.0 +2 V.sub.2)/3, (2 V.sub.2 +V.sub.5)/3, (V.sub.2 +2 V.sub.5)/3, and (2 V.sub.5 +V.sub.7)/3 can be obtained from four gray-scale reference voltages V.sub.0, V.sub.2, V.sub.5, and V.sub.7 which are supplied from external voltage sources. Accordingly, eight gray scales are realized using only four gray-scale reference voltages.
FIG. 4 shows, by way of example, the waveform of a signal voltage V.sub.1 (represented by a solid line) which is output to a data line from the circuit 30 of FIG. 3, and the waveform of a signal voltage V.sub.COM (represented by a broken line) applied to a common electrode (not shown) of a liquid crystal panel which is driven by this conventional data driver in accordance with a known alternating driving method. It is assumed in FIG. 4 that the entire driving circuit operates under the ideal condition of no load. The signal voltage V.sub.1 is one of the four interpolated voltages described above, which is produced from the gray-scale reference voltages V.sub.0 and V.sub.2 in the case where the value of the digital video data is 1. As shown in FIG. 4, the signal voltage V.sub.1 periodically oscillates between the two gray-scale reference voltages V.sub.0 and V.sub.2 in such a manner that the ratio of total time for V.sub.0 to that for V.sub.2 in one output period is 1:2. This conventional data driver operates in accordance with a so-called "line inversion method" in which the polarity of signal voltages is changed from positive to negative or vice versa at the beginning of each horizontal period, thereby preventing the deterioration of the liquid crystal display apparatus. One output period is usually set equal to one horizontal period.
FIG. 5 shows the waveforms of the gray-scale reference voltages V.sub.0 and V.sub.2, for comparison with the oscillating voltage V.sub.1 shown in FIG. 4.
FIG. 6 shows a power supply circuit 63 for supplying the gray-scale reference voltage V.sub.0 to the data driver and a power supply circuit 64 for supplying the gray scale reference voltage V.sub.2 to the data driver. The power supply circuits 63 and 64 include operational amplifiers 61 and 62, respectively. In the circuit 30 shown in FIG. 3, the gray-scale reference voltages V.sub.0 and V.sub.2 are supplied to the analog switches 34 and 35, respectively. The circuit 30 generates the oscillating voltage V.sub.1 by switching the ON-state and the OFF-state of the analog switches 34 and 35 in accordance with the control signal output from a selective control circuit 33.
FIG. 7 shows a waveform of the gray-scale reference voltage V.sub.0 in the case where all of the plurality of circuits 30 output the oscillating voltages V.sub.1 in successive horizontal periods. For example, in a VGA type liquid crystal panel, the number of the circuits 30 per scanning line is: 640.times.3 (RGB)=1920. Assuming that in a certain horizontal period, the oscillating voltages V.sub.1 are output from all of the circuits 30 corresponding to one of a plurality of scanning lines in the liquid crystal panel. This corresponds to the fact that a straight line is drawn from one end to the other end of the liquid crystal panel with gray scales corresponding to the voltage level of the oscillating voltages V.sub.1. Assuming that in another subsequent horizontal period, the oscillating voltages V.sub.1 are output from all of the circuits 30 corresponding to another one of the plurality of scanning lines in the liquid crystal panel. This corresponds to the fact that another straight line is drawn from one end to the other end in the liquid crystal panel with gray scales corresponding to the voltage level of the oscillating voltages V.sub.1.
As shown in FIG. 7, voltage changes, like a whisker shape, occur in the gray-scale reference voltage V.sub.0. The reason for the voltage changes is as follows:
When the analog switches 34 and 35 shown in FIG. 3 are switched between the ON-state and the OFF-state, there exists time when both of the analog switches 34 and 35 are in the ON-state. This causes a through current flowing between the power supply circuits 63 and 64. Such a noise component in the gray-scale reference voltage V.sub.0 (i.e., the voltage changes of the gray-scale reference voltage V.sub.0) varies depending upon how many circuits 30 output the oscillating voltage V.sub.1. In the above-mentioned VGA type liquid crystal panel, the number of the circuits 30 which output the oscillating voltage V.sub.1 is in the range of 0 to 1920 per scanning line.
In the case where the level of the noise component of the gray-scale reference voltage depends upon an image, the noise component possibly changes the average value of the gray-scale reference voltage. The change of the average value of the gray-scale reference voltage may cause deterioration of a whole image such as shadowing. Moreover, the flow of the through current between the power supply circuits 63 and 64 leads to the increase in unnecessary power consumption.
The above-mentioned problems apply to other gray-scale reference voltages in a similar manner.
When a problem arises that the above-mentioned noise component appears in a voltage which is supplied from a voltage source, one of the typical solutions is that a capacitor is provided to prevent the occurrence of the noise component. However, not any capacitor can be used for the gray-scale reference voltage sources such as power supply circuits 63 and 64 shown in FIG. 6. This is because the capacitor itself becomes a load of the voltage source which should output a signal voltage having a rectangular waveform.